Lithium aluminosilicate ceramic

ABSTRACT

A ceramic article which consists essentially, by weight on the oxide basis, of 10-25% SiO 2 , 65-85% Al 2 O 3 , and 2-12% Li 2 O and comprises beta-eucryptite as a first phase having a negative component in thermal expansion and a melting point T m1 , and a second phase having a positive component in thermal expansion which is higher than the component in thermal expansion of the first phase and a melting point T m2 , wherein T m2 &gt;T m1 , wherein the first phase is at most 50% by weight of the ceramic, and wherein the ceramic is characterized by microcracking. T m2  is at least 1800° C. The ceramic article exhibits a near zero coefficient of thermal expansion from room temperature to 800° C., a high refractoriness, and a high resistance to thermal shock properties which make the inventive ceramic extremely desirable in high temperature applications, such as filters for diesel exhaust engines.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/237,178, filed Oct. 2, 2000, entitled “LithiumAluminosilicate Ceramic”, by Beall et al.

BACKGROUND OF THE INVENTION

[0002] The instant invention relates to ceramic bodies or structureshaving compositions within the Li₂O—Al₂O₃—SiO₂ (lithium aluminosilicate)system. Specifically, the present invention relates to lithiumaluminosilicate ceramics having a low coefficient of thermal expansion(CTE), high heat capacity, high refractoriness, and high thermal shockresistance.

[0003] In the industry cordierite (2MgO-2Al₂O₃-5SiO₂) has been thecost-effective material of choice for high temperature filteringapplications, such as flow-through and wall-flow filters, due to itscombination of good thermal shock resistance, filtration efficiency, anddurability under most operating conditions.

[0004] However, under certain circumstances cordierite filters aresusceptible to damage and have even catastrophically failed.

[0005] A need therefore exists for a ceramic suitable for hightemperature filtering applications without the shortfalls of cordierite.

[0006] The present invention provides such a ceramic and a method offabricating the same.

SUMMARY OF THE INVENTION

[0007] The instant invention is founded upon the discovery of apredominately two-phase ceramic within the Li₂O—Al₂O₃—SiO₂ system whichhas high refractoriness, high resistance to thermal shock, and high heatcapacity properties which make the inventive ceramic extremely desirablein high temperature applications, such as filters for diesel exhaustengines.

[0008] Specifically the invention is a ceramic article which consistsessentially, by weight on the oxide basis, of 10-25% SiO₂, 65-85% Al₂O₃,and 2-12% Li₂O and is composed of a first phase having anisotropicthermal expansion behavior (widely differing expansions along thecrystallographic axes) with an average coefficient of thermal expansionfrom room temperature to 1000° C. of −5×10⁻⁷/° C. and being less than50% by weight of the ceramic article, and a second phase having a highermelting point than the melting point of the first phase. The meltingpoint of the second phase is preferably at least 1800° C.

[0009] The inventive ceramic structures have 32 to 50 weight % ofbeta-eucryptite (LiAlSiO₄) as a first phase having a melting pointT_(m1), and 50 to 68 weight % of a second phase having a positivecomponent in thermal expansion which is higher than the component inthermal expansion of the first phase and a melting point T_(m2), whereinT_(m2)>T_(m1). The second phase is selected from the group consisting oflithium aluminate spinel (LiAl₅O₈), lithium aluminate (LiAlO₂), corundum(Al₂O₃), and combinations thereof.

[0010] The inventive ceramic structures exhibit a coefficient of thermalexpansion (CTE) from room temperature to 800° C. of −30×10⁻⁷/° C. to+30×10⁻⁷/° C., preferably −20×10⁻⁷/° C. to +10×10⁻⁷/° C.; a permeabilityof at least 0.5×10⁻¹² m², preferably 1.0×10⁻¹² m² to 5.0×10⁻¹² m²; atotal porosity of 35-65%, preferably 45-55%; a median pore size of 8-25micrometers, preferably 15-20 micrometers; and, a high refractoriness attemperatures of between 1550° C. to 1650° C.

[0011] The inventive ceramic structures are suitable in high temperatureapplications such as filters for diesel exhaust and automotive catalyticconverters. In particular the inventive structure is especially suitableas a honeycomb diesel particular filter having an inlet end and anoutlet end and a multiplicity of cells extending from the inlet end tothe outlet end, the cells having porous walls, wherein part of the totalnumber of cells at the inlet end are plugged along a portion of theirlengths, and the remaining part of cells that are open at the inlet endare plugged at the outlet end along a portion of their lengths, so thatan engine exhaust stream passing through the cells of the honeycomb fromthe inlet end to the outlet end flows into the open cells, through thecell walls, and out of the structure through the open cells at theoutlet end.

[0012] The invention is also a method of making the ceramic article. Amixture of lithium carbonate, alumina, clay and/or sand, solvent,optionally binders, lubricants and plasticizers are formed into aplasticized batch, shaped into a green body, optionally dried, and firedat temperatures of 1300° C.-1400° C. and for a time sufficient to formthe product structure.

DETAILED DESCRIPTION OF INVENTION

[0013] The invention is a ceramic which is largely biphasic, having as afirst phase a low CTE phase and as a second phase a high meltingtemperature phase (the high temperature phase may include more than asingle phase as further described herein below). This unique phaseduality renders the inventive structure, highly refractory with anear-zero CTE, thus making it suitable for high temperature applicationssuch as filtering of particulate matter from diesel exhaust streams.

[0014] The inventive composition area lies within the Li₂O—Al₂O₃—SiO₂(LAS) system and consists essentially, by weight on the oxide basis, ofabout 10-25 SiO₂, 65-85 Al₂O₃, and 2-20 Li₂O. The preferredcompositional area consists essentially, by weight on the oxide basis,of about 13-20 SiO₂, 70-80 Al₂O₃, and 3.5-10 Li₂O. Minor amounts ofother refractory oxides, such as ZrO₂, Cr₂O₃, V₂O₃, and Ta₂O₅ mayoptionally be present.

[0015] In a preferred embodiment the inventive structure includes 32 to50% by weight a first phase of beta-eucryptite having a melting pointT_(m1), and 50 to 68% by weight a second phase having a positivecomponent in thermal expansion which is higher than the component inthermal expansion of the first phase and a melting point T_(m2), whereinT_(m2)>T_(m1).

[0016] The low CTE phase is beta-eucryptite (LiAlSiO₄) which has anaverage CTE from room temperature to 1000° C. of about −5×10⁻⁷/° C., andwith a highly anisotropic CTE (i.e., widely differing expansions alongthe crystallographic axes) at the a-axis of about +80×10⁻⁷/° C. and atthe c-axis of about −170×10⁻⁷/° C.

[0017] However, beta-eucryptite also has a low melting point of about1410° C. Therefore, the amount of beta-eucryptite in the final body isless than about 50 percent by weight, and more preferably between about32 to 45 weight percent to insure that the effective melting temperatureof the final body is not compromised. In other words the majority of theceramic is composed of the high temperature phase.

[0018] The high temperature phase has a melting point higher than thatof beta-eucryptite, preferably higher than 1800° C. The high temperaturephase is selected from the group consisting of lithium aluminate spinel(LiAl₅O₈), lithium aluminate (LiAlO₂), corundum (Al₂O₃), andcombinations thereof. Lithium aluminate spinel has a melting point ofabout 1960° C. Corundum has a melting point of about 2020° C. LiAlO₂ hasa melting point of about 1850° C.

[0019] All three of these phases have a high CTE. Lithium aluminatespinel has a CTE from room temperature to 1000° C. of about 85×10⁻⁷/°C., while corundum has a CTE from room temperature to 1 000° C. of84×10⁻⁷/° C. It is preferred that the second high temperature phase belithium aluminate spinel because it is in thermodynamic equilibrium withLiAlSiO₄ in the solid state, and also forms a rigid network incombination with liquids near this composition in the partially moltenstate. Therefore, in an especially preferred embodiment the inventiveceramic comprises about 35 weight % beta-eucryptite and 65 weight %lithium aluminate spinel.

[0020] The large CTE mismatch between the beta-eucryptite phase and thehigh temperature phase promotes microcracking either along grainboundaries between beta-eucryptite crystals or between thebeta-eucryptite and the high temperature phases which leads to a CTEover a temperature range from room temperature to 800° C. of from−30×10⁻⁷/° C. to 30×10⁻⁷/° C., preferably −20×10⁻⁷/° C. to 10×10⁻⁷/° C.,resulting in excellent thermal shock resistance in the inventivestructure. Microcracked bodies tend to bias the CTE towards the mostnegative CTE component because the opening of microcracks on coolingaccommodates the normal positive components.

[0021] In addition the inventive structure exhibits high refractorinessat temperatures of 1550° C. to 1650° C. Refractoriness is a measure ofthe deformation in the structure when exposed to high temperatures suchas above 1500° C., for a period of time in duration about 10 hours. Theextremely high refractoriness in the inventive structure is believed tobe the result of the spinel framework maintaining continuity and thebeta-eucryptite rich melt attaching itself to the spinel network.

[0022] Another advantage in the inventive structure is high permeabilityby virtue of high, interconnected porosity and large median pore size.The permeability is at least about 0.5×10⁻¹² m², and preferably betweenabout 1.0×10¹² m² to 5×10⁻¹² m². Permeability is a measure of how easilya fluid can flow through a porous structure. At a constant temperatureand fluid viscosity the permeability depends on the percent openporosity, pore size and how well connected the pores are to one another.

[0023] The open porosity is between about 35-65%, by volume, andpreferably between about 45-55%, by volume. The median pore size isbetween about 8-25 micrometers, and preferably between about 15-20micrometers to maintain good filtration efficiency. Open porosityreported as volume percent and pore size reported as median porediameter in micrometers are measured by mercury porosimetry.

[0024] The invention also relates to a method for fabricating theinventive LAS structure. A mixture is formed from raw materials whichinclude lithium carbonate, an alumina-forming source, a silica-formingsource and/or kaolin selected to form a composition which consistsessentially, by weight on the oxide basis, of about 10-25 SiO₂, 65-85Al₂O₃, and 2-20 Li₂O, and preferably about 13-20 SiO₂, 70-80 Al₂O₃, and3.5-10 Li₂O. Table 1 reports examples of compositions and the resultingphase assemblages according to the present invention.

[0025] Raw materials are blended together with organic constituents thatmay include plasticizers, lubricants, binders, and solvents. Water mayalso optionally be added as a solvent. The mixture is shaped into agreen body, optionally dried, and then fired at a temperature and for atime sufficient to form the final product structure.

[0026] The alumina forming source is a powder which, when heated to asufficiently high temperature in the absence of other raw materials,yields substantially pure aluminum oxide, and includes alpha-alumina, atransition alumina such as a gamma-alumina or rho-alumina, boehmite,aluminum hydroxide, and mixtures thereof. Alpha-alumina is preferred.

[0027] The particle size of alumina-forming source particle size has tobe sufficiently large for microcraking to be induced in the finalstructure, and small enough for good extrusion to occur. The hightemperature phase inherits the grain size and morphology from thealumina-forming source. Accordingly the particle size of thealumina-forming source has to be at least 10 micrometers and no greaterthan 50 micrometers, preferably between about 15 to 25 micrometers;single crystal particles below 10 micrometers there would result ininsufficient strain along grain boundaries with adjacent differing CTEto develop microcracking; single crystal particles above 50 micrometerswould result in large microcracks which may extend during thermalcycling across webs. The morphology of the alumina source is alsoimportant and has to be macrocrystalline with no aggregates of finecrystallites.

[0028] The silica-forming source includes, but is not limited to,quartz. Optionally, kaolin (which aids in the extrusion process) may beadded, preferably in an amount no greater than 20% by weight.

[0029] The inventive structure is particularly suitable for hightemperature filtration applications. In particular the inventivestructures are particularly suitable for diesel particulate filterapplications. For such applications the raw material mixture ispreferably shaped by extrusion into a honeycomb multicellular structure,as known in the art.

[0030] The resulting shaped green honeycomb bodies are usually dried andheated to a maximum temperature of about 1300-1400° C. over a period ofabout 28 hours, and held at the maximum temperature for about 6-10hours.

[0031] While the construction of the filter can have any shape orgeometry suitable for a particular application, it is preferred that itbe multicellular structures such as a honeycomb structures. Thehoneycomb structure has an inlet and outlet end or face, and amultiplicity of cells extending from the inlet end to the outlet end,the cells having porous walls. The inventive filters have cellulardensities from about 100 cells/in² (15.5 cells/cm²) to about 400cells/in² (62 cells/cm²).

[0032] To obtain a filtering device, a portion of the cells of thehoneycomb at the inlet end or face are plugged, as known in the art. Theplugging is only at the ends of the cells which is typically to a depthof about 5 to 20 mm, although this can vary. A portion of the cells onthe outlet end but not corresponding to those on the inlet end areplugged. Therefore, each cell is plugged only at one end. The preferredarrangement is to have every other cell on a given face plugged as in acheckered pattern.

[0033] An advantage of the diesel particulate filters of the presentinvention have many advantages is a low pressure drop across the lengthof the filter and low back pressure against the engine comparable tocommercially available SiC counterparts. The pressure drop across thefilter is a function of the accumulation of the carbonaceous soot on thewalls of the diesel particulate filter. As amount of soot accumulatedincreases, it creates a progressive increase in the resistance to flowof the exhaust gas through the walls of the filter and carbon sootlayer. This resistance to flow is manifested as a pressure drop that canbe measured across the length of the filter, and results in an increasedback pressure against the engine.

[0034] Although the preferred application is for diesel particulatefilters, it is to be noted that the inventive ceramic is equallysuitable as automotive flow-through substrates.

EXAMPLES

[0035] To more fully illustrate the invention, the followingnon-limiting examples of extruded honeycombs are presented in Tables 2and 3. All parts, portions and percentages are on a basis of total rawmaterials weight unless otherwise stated.

[0036] The combinations of lithium carbonate, quartz, alumina and kaolinshown in Table 2, are blended together with about 3 to 7 parts of methylcellulose. Steric acid is added as a lubricant at about 0.5 to 1 part.The particle profiles of the raw materials are also shown in the Table2.

[0037] Subsequently about 20 to 30 parts of deionized water aregradually added to each powder mixture in a muller. After kneading, thecombined ingredients are extruded through a die into honeycomb bodieshaving approximately 100 to 200 cells per square inch and having a wallthickness of about 0.010 to 0.025 inch. The bodies thus formed are cutto desired lengths and heated in an oven at 95° C. until dried.

[0038] The samples are fired in an electrically heated furnace at ratesranging from 20° C./hr to 100° C./hr over various temperature intervalsto a maximum temperature of 1350 to 1400° C. for a period of 28 hours,with a hold at the maximum temperature for about 6-10 hours to developthe final product structure, and cooled by shutting off power to thefurnace, as presented in Table 3.

[0039] Table 3 reports physical properties as determined on theexamples. Porosities and pore sizes of selected samples werecharacterized by mercury porosimetry. Total porosity is reported involume percent and pore sizes in micrometers. The permeability wasdetermined as described above and is reported in 10⁻¹² m². Meancoefficients of thermal expansion from 22 to 800° C. were measured usinga dilatometer and are reported in 10⁻⁷/° C.

[0040] Examples 1 through 7 have 35% by weight (of the total body)beta-eucryptite and 65% by weight (of the total body) lithium aluminatespinel. Example 8 has 30% by weight beta-eucryptite and 70% by weightlithium aluminate spinel. At 30% by weight beta-eucryptite, Example 8displays a thermal expansion coefficient greater than 30×10⁻⁷/° C.,which is undesirable in diesel particulate filtration. Therefore,preferably for such applications beta-eucryptite is at a level ofbetween 32%. Example 8 also displays unacceptable levels ofpermeability, total porosity and median pore size.

[0041] In addition to Example 8, Examples 1, 2, and 3 are comparativebecause these examples also have thermal expansion coefficients greaterthan 30×10⁻⁷/° C. The unacceptably high thermal expansion coefficientsin these example result from the use of an alumina-forming source havinga particle size distribution outside of the range of 10 to 50micrometers. Example C which has an alumina median particle size of 60micrometers exhibited an unusually large CTE of 63.5×10⁻⁷/° C. Uponfurther analysis it was discovered that although the median particlesize was determined to be 60 micrometers, the alumina grains of thatparticular alumina-source were about 1 micrometer. In fact, theindividual alumina particles were an agglomeration of a plurality ofthese 1 micron alumina grains. Therefore, in the present invention it isimportant that the alumina-source have particles which are individualgrains between 10 and 50 micrometers, and preferably between about 15 to25 micrometers.

[0042] An advantage of the present inventive filtering structures isdecreased reaction with metal oxide “ash” particles that are carried byexhaust gas in automobile engines. Metal oxide “ash” particles are notcombustible and, therefore, are not removed during regeneration. Aproblem which exists in the industry is that if temperatures during theregeneration process reach sufficiently high values, the ash may sinterto the filter material or even react with the filter material resultingin partial melting.

[0043] Inventive bodies were brought into contact with metal oxide ashand heater to about 1200° C. There was no obvious (observable) reaction;in contrast, in commercially available cordierite filters under similartesting conditions there was sintering and melting at thesetemperatures.

[0044] It should be understood that while the present invention has beendescribed in detail with respect to certain illustrative and specificembodiments thereof, it should not be considered limited to such but maybe used in other ways without departing from the spirit of the inventionand the scope of the appended claims. TABLE 1 Oxide Composition (wt. %)A B C D E SiO₂ 19.1 21.5 23.9 16.7 16.7 Al₂O₃ 72.7 70.1 67.4 75.6 79.6Li₂O 8.2 8.4 8.7 7.7 3.8 Phase LiAlSiO₄ LiAlSiO₄ LiAlSiO₄ LiAlSiO₄LiAlSiO₄ Assemblage LiAl₅O₈ LiAl₅O₈ LiAl₅O₈ LiAl₅O₈ ss LiAl₅O₈ Al₂O₃Oxide Composition (wt. %) F G H I J SiO₂ 16.7 14.4 16.7 16.7 16.7 Al₂O₃73.6 83.2 68.1 71.9 64.4 Li₂O 9.8 2.4 15.2 11.5 18.9 Phase LiAlSiO₄LiAlSiO₄ LiAlSiO₄ LiAlSiO₄ LiAlSiO₄ Assemblage LiAl₅O₈ ss LiAl₅O₈LiAl₅O₈ LiAlO₂ mLiAlO₂ Al₂O₃ LiAlO₂ LiAlO₂ mLiAl₅O₈

[0045] TABLE 2 Example Number  1    2    3    4    5    6    7    8  Example Type comp. comp. comp. inv. inv. inv. inv. comp. Raw MaterialComposition (Median Particle Size (micrometers)) Li₂CO₃ (6) 17.2  17.2 17.2  17.2  17.2  17.2  17.2  16.91 Quartz 1 (6) — 14.96 14.96 — — — — —Quartz 2 (17) — — — 14.96 — — — — Quartz 3 (17) — — — — 14.96 14.96 6.89  8.83 Alumina 1 (5) 53.88 67.8  — — — — — — Alumina 2 (60*) — —67.8  — — — — — Alumina 4 (15) — — — 67.8  67.8  67.8  59.69 66.41Kaolin 1 (1) — — — — — — 16.61  8.4  Kaolin 2 (4) 16.13 — — — — — — —Calcined Kaolin 1 (1) 13.19 — — — — — — —

[0046] TABLE 3 Example Number 1 2 3 4 5 6 7 8 Example Type comp. comp.comp. inv. inv. inv. inv. comp. Oxide Composition (wt. %) SiO₂ 16.6716.67 16.67 16.67 16.67 16.67 16.67 14.29 Al₂O₃ 75.58 75.58 75.58 75.5875.58 75.58 75.58 78.08 LiO₂ 7.75 7.75 7.75 7.75 7.75 7.75 7.75 7.43Phase Assemblage Beta-Eucryptite 35 35 35 35 35 35 35 30 (wt. %) LithiumAluminate 65 65 65 65 65 65 65 70 Spinel (wt. %) Properties Mean GTE(22- 55 50.3 63.5 −3.5 −15.8 9.8 −1.8 32.0 800° C.) (10⁻⁷/° C.)Permeability (10⁻¹² — — — — 4.5 3.5 1.4 44.6 m²) Total Porosity (% — 2534.8 34.8 55.2 38.1 47.2 14.2 vol.) Median Pore Size — 6 13 13 23 22 161.3 (micrometers)

It is claimed:
 1. A ceramic comprising beta-eucryptite (LiAlSiO₄) as afirst phase having a negative component in thermal expansion and amelting point T_(m1), and a second phase having a positive component inthermal expansion which is higher than the component in thermalexpansion of the first phase and a melting point T_(m2), whereinT_(m2)>T_(m1), wherein the first phase is at most 50% by weight of theceramic, and wherein the ceramic is characterized by microcracking. 2.The ceramic of claim 1 wherein the second phase has a T_(m2) of at least1800° C.
 3. The ceramic of claim 2 wherein the second phase is selectedfrom the group consisting of lithium aluminate spinel (LiAl₅O₈), lithiumaluminate (LiAlO₂), corundum (Al₂O₃), and combinations thereof.
 4. Theceramic of claim 3 wherein the second phase is lithium aluminate spinel(LiAl₅O₈).
 5. The ceramic of claim 1 wherein the first phase isbeta-eucryptite (LiAlSiO₄) and the second phase is lithium aluminatespinel (LiAl₅O₈).
 6. The ceramic of claim 5 wherein the beta-eucryptiteis 32-50% by volume and lithium aluminate spinel is 50-68% by volume. 7.The ceramic of claim 6 wherein the beta-eucryptite is 35-40 weight % andlithium aluminate spinel is 55-60 weight %.
 8. The ceramic of claim 7wherein the beta-eucryptite is 35 weight % and lithium aluminate spinelis 65 weight %.
 9. The ceramic of claim 1 wherein the ceramic has a meancoefficient of thermal expansion is −30×10⁻⁷/° C. to +30×10⁻⁷/° C. fromroom temperature to 800° C.
 10. The ceramic of claim 9 wherein theceramic has a mean coefficient of thermal expansion is −20×10⁻⁷/° C. to+10×10⁻⁷/°C. from room temperature to 800° C.
 11. The ceramic of claim10 wherein the ceramic has a permeability of at least 0.5×10⁻¹² m². 12.The ceramic of claim 11 wherein the permeability is 1.0-5.0×10⁻¹² m².13. The ceramic of claim 12 wherein the ceramic has a median pore sizeof between 8-25 micrometers.
 14. The ceramic of claim 13 wherein theceramic has a median pore size of between 15-20 micrometers.
 15. Theceramic of claim 14 wherein the ceramic has a total porosity of at leastbetween 35-65% by volume.
 16. The ceramic of claim 15 wherein the totalporosity is between 45-55% by volume.
 17. A structure which consistsessentially, by weight on the oxide basis, of 10-25% SiO₂, 65-85% Al₂O₃,and 2-12% LiO₂, having a first phase of beta-eucryptite (LiAlSiO₄) and asecond phase selected from the group consisting of lithium aluminatespinel (LiAl₅O₈), lithium aluminate (LiAlO₂), corundum (Al₂O₃), andcombinations thereof, wherein the first phase is at most 50% by weightof the structure and the structure is characterized by microcracking.18. The structure of claim 17 consisting essentially, by weight on theoxide basis, of 13-20% SiO₂, 70-80% Al₂O₃, and 3.5-10% Li₂O.
 19. Thestructure of claim 17 wherein the first phase is beta-eucryptite(LiAlSiO₄) and the second phase is lithium aluminate spinel (LiAl₅O₈).20. The structure of claim 19 wherein the beta-eucryptite is 32-50% byvolume and lithium aluminate spinel is 50-68% by volume.
 21. Thestructure of claim 20 wherein the beta-eucryptite is 35% by volume andlithium aluminate spinel is 65% by volume.
 22. The structure of claim 17wherein the structure has a mean coefficient of thermal expansion of−30×10⁻⁷/° C. to 30×10⁻⁷/° C. from room temperature to 800° C.
 23. Thestructure of claim 22 wherein the structure has a permeability of atleast 0.5×10⁻¹² m².
 24. The structure of claim 23 wherein thepermeability is 1.5-5×10⁻¹² m².
 25. The structure of claim 24 whereinthe structure has a median pore size of between 8-25 micrometers. 26.The structure of claim 25 wherein the structure has a total porosity ofat least between 35-65% by volume.
 27. The structure of claim 26 whereinthe total porosity is between 45-55% by volume.
 28. The structure ofclaim 27 wherein the structure is used as a wall-flow diesel enginefilter.
 29. The structure of claim 28 having the shape of a honeycomb,the honeycomb having an inlet end and an outlet end, and a multiplicityof cells extending from the inlet end to the outlet end, the cellshaving porous walls, wherein part of the total number of cells at theinlet end are plugged along a portion of their lengths, and theremaining part of cells that are open at the inlet end are plugged atthe outlet end along a portion of their lengths, so that an engineexhaust stream passing through the cells of the honeycomb from the inletend to the outlet end flows into the open cells, through the cell walls,and out of the structure through the open cells at the outlet end.
 30. Adiesel particulate filter comprising a ceramic article which consistsessentially, by weight on the oxide basis, of 10-25% SiO₂, 65-85% Al₂O₃,and 2-12% Li₂O and has a first phase of beta-eucryptite (LiAlSiO₄) and asecond phase selected from the group consisting of lithium aluminatespinel (LiAl₅O₈), lithium aluminate (LiAlO₂), corundum (Al₂O₃), andcombinations thereof, wherein the first phase is at most 50% by weightof the ceramic, and wherein the filter has the shape of a honeycomb,wherein the honeycomb has an inlet end and an outlet end and amultiplicity of cells extending from the inlet end to the outlet end,the cells having porous walls, wherein part of the total number of cellsat the inlet end are plugged along a portion of their lengths, and theremaining part of cells that are open at the inlet end are plugged atthe outlet end along a portion of their lengths, so that an engineexhaust stream passing through the cells of the honeycomb from the inletend to the outlet end flows into the open cells, through the cell walls,and out of the structure through the open cells at the outlet end.
 31. Amethod of making a ceramic comprising: a) selecting raw materials toform a composition which forms a ceramic with a Li₂O—Al₂O₃—SiO₂ system,the composition consisting essentially, by weight on the oxide basis, of10-25% SiO₂, 65-85% Al₂O₃, and 2-12% Li₂O, the raw materials beingcomposed of: lithium carbonate; an alumina-forming source having a meanparticle size of between 10 and 50 micrometers; and, a silica-formingsource; b) blending the raw materials to form a plasticized mixture; c)shaping the plasticized mixture into a green body; d) firing the greenbody to produce a ceramic including a first phase having a first phasehaving a negative component in thermal expansion, and a second phasehaving a melting point above 1800° C. and a positive component inthermal expansion and being higher than the component in thermalexpansion of the first phase, wherein the first phase is less than 50%by weight of the ceramic, and the microstructure of the ceramic ischaracterized by microcracking.
 32. The method of claim 21 wherein thealumina-forming source has a mean particle size of between 15-25micrometers.
 33. The method of claim 31 wherein the alumina-formingsource is selected from the group consisting of aluminum oxide,alpha-alumina, gamma-alumina, rho-alumina, boehmite, aluminum hydroxide,and mixtures thereof.
 34. The method of claim 31 wherein thesilica-forming source is quartz.
 35. The method of claim 31 wherein theshaping is done by extrusion.
 36. The method of claim 31 wherein thefiring step is carried out at 1300-1400° C. over a period of 28 hourswith a hold time of 6-10 hours.